Back in the day… before bayonet mounts… before canon mount lenses couldn’t be put on nikon bodies and nikon mount lenses couldn’t be put on canon bodies there were a lot of cameras and lenses from multiple manufactures that used a threaded mount called the m42 mount. I don’t know what the m was for but the 42 was because the threads were 42mm (I think). This was a strong standard and a lot of these lenses were made in a lot of variations and there are still a lot of them out there. Because these lenses had threaded mounts (I think) and because they were made before auto focus they are very easy to adapt to be put on modern camera bodies.
The lenses range from dirt cheap to killer deal. The quality of the glass can be as good or better than modern lenses. The adapters range from 5 bucks to maybe 50 with varying functionality. I’ve got a $5 adapter and a pentax 50mm 1.4 SMC so I’ll speak specifically about that combo. I got the lens on ebay for 60 bucks so I’m at $65 total for a very sharp, very fast lens. The modern canon equivalent is $400. I love it. The only down side is no auto focus and I’m perfectly happy to trade that for $335 off a killer fast lens (not to mention that the more expensive adapters may have more functionality).
So if you’re looking more info or searching for adapters and lenses on ebay use terms like m42, 1.4, eos, adapter… etc. Make sure you get familiar with your specific application because different adapters and different lenses will vary slightly in terms of cost, functionality and features. Oh and one other thing… some of them are radioactive so don’t carry them in your pocket or sleep with them under your pillow. Have a nice day.